In typical compact disks (CDs), a region referred to as an IFPI (International Federation of the Phonographic Industry) code is recorded in an area further towards the center than the area in which signals used by a user, such as an audio signal and a TOC (Table Of Contents), are recorded. This code is imprinted with markings of vendors, manufacturing plants, disk numbers, etc., for the purpose of preventing illegal copies called pirated copies.
The recorded markings of vendors, manufacturing plants, disk numbers, etc., are visually recognizable. In playing back a CD or the like, such markings are not readable on a playback apparatus.
That is, the content of such an IFPI code cannot be reflected in operation control of the playback apparatus. Moreover, as described above, since the content can be visually confirmed, the IFPI code has a problem in that the code itself can be copied with ease.
Recordable optical disks onto which a user is able to readily duplicate CDs, DVDs, etc., have also been commercially available. For example, the use of commercially available CD-R disks and apparatuses allows digital information recorded in a CD to be copied exactly by a simple operation. Such copies can be copied with violating copyright, protected under the copyright law, and another problem occurs in that the copyright owner can get no fair reward.
In order to overcome the foregoing problems, in the related art, assuming that information used by a user, such as an audio signal, is set as first information, second information serving as information for copyright protection or the like is superimposed and recorded on the first information.
In the related art, for example, there has been proposed an optical disk in which a plurality of specific pit sequences, each being configured so that a portion of multiple information pits is arranged so that the center line thereof is displaced in the radial direction, are arranged on an information track in units of data blocks of a recording signal at certain intervals in an intermittent manner.
Another example has been proposed, i.e., an optical disk equipped with an illegal copy preventing function, in which, in a portion of a specific radial area of the disk, the information pit sequences are displaced in the radial direction of the track from the center line of the information pit sequence across the track length where a signal variation detectable in a tracking error signal band occurs (see, e.g., Japanese Unexamined Patent Application Publication No. 9-81938).
Furthermore, in the related art, there has been proposed an optical recording carrier in which first information serving as main information and second information relating to control information for playing back the first information are recorded, wherein third information that is different from the first and second information is recorded in association with the second information such that when the first information is played back based on the second information, the third information is not directly contained in the decoded first information (see Japanese Unexamined Patent Application Publication No. 11-66739).
Furthermore, in the related art, there has been proposed a data recording medium in which first data is recorded by repeating pits and lands for a duration equal to an integer multiple of the duration corresponding to a predetermined basic period, wherein second data is recorded by displacing the pits in the direction perpendicular to the track direction from the center of the track and the displacement is within a predetermined amount as long as an off-track situation does not occur (see Japanese Unexamined Patent Application Publication No. 2000-242929).
Furthermore, in the related art, there has been proposed an optical recording medium in which a recording track is formed of continuous lands or grooves and pits are formed on the recording track based on main data, thereby recording the main data, wherein the pits formed on the recording track a predetermined distance apart are displaced by a predetermined amount in the direction orthogonal to the center line of the recording track in the track direction based on additional data, thereby recording the additional data (see Japanese Unexamined Patent Application Publication No. 2002-197671).
In this publication, claim 5 recites that the amount of displacement ranges from 20 nm to 100 nm.
According to the above-noted publications of the related art, second information is recorded by laterally displacing (wobbling) pits. Such information recording by wobbles is only feasible on a mastering apparatus that irradiates light onto a master optical disk. It is therefore impossible to record information recorded by wobbles onto a recordable apparatus such as a CD-R. The wobbled information is recorded onto a master disk, and the presence of the information is checked during playback. Thus, illegal disks copied by a recordable apparatus such as a CD-R can be eliminated.
However, the publications of the related art have a problem in that a sufficient amount of wobbling is required to ensure that information recorded by the wobbles is detected. This is because the signal for detecting the wobbles has a poor signal-to-noise ratio (SNR).
For example, in Japanese Unexamined Patent Application Publication No. 2002-197671 of the related art, the wobble amount ranges from 20 nm to 100 nm, and it is necessary to manufacture a disk with a relatively large amount of wobble. Such large displacement can cause wobbles to be observed and confirmed using an electron microscope or the like. Therefore, a problem occurs in that the content of the wobbles can be analyzed, and the risk of illegal copying increases.
In general, one conceivable technique to overcome a low SNR is to extend the wobbled area, i.e., a pit sequence, and to remove noise using a low-pass filter. However, an actual optical disk playback apparatus is designed so that a tracking servo is used to adjust the position of a light spot to the center of a wobble. If the wobbled area is extended, the amplitude of a wobble detection signal is gradually lowered according to movement of the spot, and the wobble detection signal has substantially no amplitude at the position where the movement of the light spot is completed.
In view of such functionality of the tracking servo, it is not effective to merely extend the wobbled area. This is because a large wobble amplitude is required in the related art.
The present applicant has proposed an improvement, that is, an optical disk onto which main data is recorded according to the length and pitch of pits along a track and sub-data is recorded by displacing the pits in the inner and outer radial directions with respect to the center of the track, wherein the pits are displaced according to a result of computation of the sub-data and a predetermined binary sequence (see Japanese Unexamined Patent Application Publication No. 2000-195049).
In this optical disk, information recorded by wobbles is detected by integral detection using a predetermined binary sequence. In this invention, one bit of copyright protection information is allocated to one frame of a CD for recording. One frame is a disk length equal to 12 mm. A playback signal for such a long period is detected by integration, thus allowing a wobble recorded with a relatively small amplitude to be sufficiently detected.
This publication of the related art can thus greatly reduce the amount of pit wobble. While overcoming the previously described drawbacks with the related art, practical copyright protection is therefore achievable.
For example, if a defect occurs at the beginning of a frame during playback of an optical disk, a clock variation phenomenon (bit slip) can occur.
In the Japenese Patent Publication No. 2000-195049, if a bit slip occurs, as described above, the clock becomes out of phase, and the binary sequence used in recording is not reproduced and correct data decoding is thus impossible. The bit slip is not overcome until the next sync pattern has been played back. Therefore, if a defect occurs at the beginning of a frame and a bit slip occurs, it is impossible to play back the bit of the data, which is troublesome.
In a case where no error correcting code is added to the sub-information, namely, copyright protection information, if a read error of one bit occurs, a serious problem occurs in that the data must be played back from the beginning.